of flat…shaped bodies： for even the wind can scarcely move them because of their power of resistance。 The same immobility； they say； is produced by the flatness of the surface which the earth presents to the air which underlies it； while the air； not having room enough to change its place because it is underneath the earth； stays there in a mass； like the water in the case of the water…clock。 And they adduce an amount of evidence to prove that air； when cut off and at rest； can bear a considerable weight。 Now； first； if the shape of the earth is not flat； its flatness cannot be the cause of its immobility。 But in their own account it is rather the size of the earth than its flatness that causes it to remain at rest。 For the reason why the air is so closely confined that it cannot find a passage； and therefore stays where it is； is its great amount： and this amount great because the body which isolates it； the earth； is very large。 This result； then； will follow； even if the earth is spherical； so long as it retains its size。 So far as their arguments go； the earth will still be at rest。 In general； our quarrel with those who speak of movement in this way cannot be confined to the parts； it concerns the whole universe。 One must decide at the outset whether bodies have a natural movement or not； whether there is no natural but only constrained movement。 Seeing； however； that we have already decided this matter to the best of our ability； we are entitled to treat our results as representing fact。 Bodies； we say； which have no natural movement； have no constrained movement； and where there is no natural and no constrained movement there will be no movement at all。 This is a conclusion； the necessity of which we have already decided； and we have seen further that rest also will be inconceivable； since rest； like movement； is either natural or constrained。 But if there is any natural movement； constraint will not be the sole principle of motion or of rest。 If； then； it is by constraint that the earth now keeps its place； the so…called 'whirling' movement by which its parts came together at the centre was also constrained。 （The form of causation supposed they all borrow from observations of liquids and of air； in which the larger and heavier bodies always move to the centre of the whirl。 This is thought by all those who try to generate the heavens to explain why the earth came together at the centre。 They then seek a reason for its staying there； and some say； in the manner explained； that the reason is its size and flatness； others； with Empedocles； that the motion of the heavens； moving about it at a higher speed； prevents movement of the earth； as the water in a cup； when the cup is given a circular motion； though it is often underneath the bronze； is for this same reason prevented from moving with the downward movement which is natural to it。） But suppose both the 'whirl' and its flatness （the air beneath being withdrawn） cease to prevent the earth's motion； where will the earth move to then？ Its movement to the centre was constrained； and its rest at the centre is due to constraint； but there must be some motion which is natural to it。 Will this be upward motion or downward or what？ It must have some motion； and if upward and downward motion are alike to it； and the air above the earth does not prevent upward movement； then no more could air below it prevent downward movement。 For the same cause must necessarily have the same effect on the same thing。 Further； against Empedocles there is another point which might be made。 When the elements were separated off by Hate； what caused the earth to keep its place？ Surely the 'whirl' cannot have been then also the cause。 It is absurd too not to perceive that； while the whirling movement may have been responsible for the original coming together of the art of earth at the centre； the question remains； why now do all heavy bodies move to the earth。 For the whirl surely does not come near us。 Why； again； does fire move upward？ Not； surely； because of the whirl。 But if fire is naturally such as to move in a certain direction； clearly the same may be supposed to hold of earth。 Again； it cannot be the whirl which determines the heavy and the light。 Rather that movement caused the pre…existent heavy and light things to go to the middle and stay on the surface respectively。 Thus； before ever the whirl began； heavy and light existed； and what can have been the ground of their distinction； or the manner and direction of their natural movements？ In the infinite chaos there can have been neither above nor below； and it is by these that heavy and light are determined。 It is to these causes that most writers pay attention： but there are some； Anaximander； for instance； among the ancients； who say that the earth keeps its place because of its indifference。 Motion upward and downward and sideways were all； they thought； equally inappropriate to that which is set at the centre and indifferently related to every extreme point； and to move in contrary directions at the same time was impossible： so it must needs remain still。 This view is ingenious but not true。 The argument would prove that everything； whatever it be； which is put at the centre； must stay there。 Fire； then； will rest at the centre： for the proof turns on no peculiar property of earth。 But this does not follow。 The observed facts about earth are not only that it remains at the centre； but also that it moves to the centre。 The place to which any fragment of earth moves must necessarily be the place to which the whole moves； and in the place to which a thing naturally moves； it will naturally rest。 The reason then is not in the fact that the earth is indifferently related to every extreme point： for this would apply to any body； whereas movement to the centre is peculiar to
earth。 Again it is absurd to look for a reason why the earth remains at the centre and not for a reason why fire remains at the extremity。 If the extremity is the natural place of fire； clearly earth must also have a natural place。 But suppose that the centre is not its place； and that the reason of its remaining there is this necessity of indifference…on the analogy of the hair which； it is said； however great the tension； will not break under it； if it be evenly distributed； or of the men who； though exceedingly hungry and thirsty； and both equally； yet being equidistant from food and drink； is therefore bound to stay where he is…even so； it still remains to explain why fire stays at the extremities。 It is strange； too； to ask about things staying still but not about their motion；…why； I mean； one thing； if nothing stops it； moves up； and another thing to the centre。 Again； their statements are not true。 It happens； indeed； to be the case that a thing to which movement this way and that is equally inappropriate is obliged to remain at the centre。 But so far as their argument goes； instead of remaining there； it will move； only not as a mass but in fragments。 For the argument applies equally to fire。 Fire； if set at the centre； should stay there； like earth； since it will be indifferently related to every point on the extremity。 Nevertheless it will move； as in fact it always does move when nothing stops it； away from the centre to the extremity。 It will not； however； move in a mass to a single point on the circumference…the only possible result on the lines of the indifference theory…but rather each corresponding portion of fire to the corresponding part of the extremity； each fourth part； for instance； to a fourth part of the circumference。 For since no body is a point； it will have parts。 The expansion； when the body increased the place occupied； would be on the same principle as the contraction； in which the place was diminished。 Thus； for all the indifference theory shows to the contrary； earth also would have moved in this manner away from the centre； unless the centre had been its natural place。 We have now outlined the views held as to the shape； position； and rest or movement of the earth。
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Let us first decide the question whether the earth moves or is at rest。 For； as we said； there are some who make it one of the stars； and others who； setting it at the centre； suppose it to be 'rolled' and in motion about the pole as axis。 That both views are untenable will be clear if we take as our starting…point the fact that the earth's motion； whether the earth be at the centre or away from it； must needs be a constrained motion。 It cannot be the movement of the earth itself。 If it were； any portion of it would have this movement； but in fact every part moves in a straight line to the centre。 Being； then； constrained and unnatural； the movement could not be eternal。 But the order of the universe is eternal。 Again； everything that moves with the circular movement； except the first sphere； is observed to be passed； and to move with more than one motion。 The earth； then； also； whether it move about the centre or as stationary at it； must necessarily move with two motions。 But if this were so； there would have to be passings and turnings of the fixed stars。 Yet no such thing is observed。 The same stars always rise and set in the same parts of the earth。 Further； the natural movement of the earth； part and whole alike； is the centre of the whole…whence the fact that it is now actually situated at the centre…but it might be questioned since both centres are the same； which centre it is that portions of earth and other heavy things move to。 Is this their goal because it is the centre of the earth or because it is the centre of the whole？ The goal； surely； must be the centre of the whole。 For fire and other light things move to the extremity of the area which contains the centre。 It happens； however； that the centre of the earth and of the whole is the same。 Thus they do move to the centre of the earth； but accidentally； in virtue of the fact that the earth's centre lies at the centre of the whole。 That the centre of the earth is the goal of their movement is indicated by the fact that heavy bodies moving towards the earth do not parallel but so as to make equal angles； and thus to a single centre； that of the earth。 It is clear； then； t